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<html><!-- Mirrored from c-faq.com/struct/align.esr3.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:28 GMT --><head><title></title></head><body>Newsgroups: comp.lang.c<br>From: Clive D.W. Feather<br>Subject: Re: Criticism of C-FAQ submission invited<br>Date: Thu, 22 Sep 1994 13:08:17 GMT<br>Message-ID: <CwJ8Hu.8y6@scone.london.sco.com><p>In article <35qdf0$7iq@netaxs.com>, Eric Raymond wrote:<br>> The first member of each<br>> continuous group of bit fields is typically word-aligned, with all following<br>> being continuously packed into following words (though ANSI C requires the<br>> latter only for bit-field groups of less than word size).<p>False. ISO C (please get the name right) requires that the first bitfield be put into some addressable object, but there is no requirementthat it be a ``word'' (thus an 11 bit field might be put into a halfwordon a 32 bit system). If the following field will fit in the same objectit must be put there. If it doesn't it can be overlapped between twoobjects or put into a new object. Thus:<p><pre> int a : 11; int b : 2; int c : 11; int d : 7;</pre><p>could be packed as:<p><pre> 32-bit object: a, b, c, d</pre><p>or as:<p><pre> 16-bit object: a, b 16-bit object: c 16-bit object: d</pre><p>or as:<p><pre> 16-bit object: a, b, 3 bits of c 16-bit object: 8 bits of c, d</pre><p>> Q: How do I figure the ``true size'' (with trailing padding) of a type when<br>> it occurs in a struct or array?<p>In a struct:<p><pre> offsetof (structure, next field) - offsetof (structure, field)</pre><p>or:<p><pre> sizeof (structure) - offsetof (structure, last field)</pre><p>There is no padding in an array.<p>> A: By rounding up its apparent size to a multiple of its alignment.<p>Not necessarily. The padding can depend on what is following.<p>> (assume a 32-bit architecture with normal C sizes<br>> of long=4, int=4, short=2, and char=1 on all these unless otherwise<br>> specified).<p>and assuming pointer=4.<p>> (4) struct {char *; short; long;} will have a half-word of padding after<br>> the short<p>or before it. On big-endian machines it may be easier to access thehalf-word in the low-order half.<p>> Q: How can I get complete control of the bit-level layout of a struct?<br>> A: This is what bit-fields are for. Declare all your members as bit fields.<br>> The structure may still have invisible trailing padding (unless your last<br>> bit field exactly fills to the end of the last machine word), but on modern<br>> architectures this will eliminate all other padding and give you complete<br>> control of the bit layout of the declared parts (albeit at a significant<br>> cost in access time). However, ANSI does <em>not</em> guarantee this well-<br>> behavedness, so look out for snags.<p>Firstly, there may be padding whenever fields don't fit exactly. Secondly,you still aren't guaranteed the order of the bit-fields within each``addressable object''.<p>> On some poorly-implemented C compilers on very old 36-bit word-oriented<br>> architectures, even this may not be enough; a bit field that runs over<br>> the end of a word boundary may force the next bit field to start at the<br>> beginning of the following word, leaving an invisible gap of up to 35 bits.<p>Or on well-implemented compilers on modern architectures that choose notto go to the effort of splitting bit-fields across word boundaries.<p>> On these machines, your only option is to declare the struct as a single<br>> char array and do the field accesses yourself in C.<p>And you still won't know the ordering of bits within the word relativeto the external format you are conforming to. This can't be solved in astrictly conforming manner.<p><pre>-- Clive D.W. Feather | Santa Cruz Operation | If you lie to the compiler,clive@sco.com | Croxley Centre | it will get its revenge.Phone: +44 1923 813541 | Hatters Lane, Watford | - Henry SpencerFax: +44 1923 813811 | WD1 8YN, United Kingdom | <= NOTE: NEW PHONE NUMBERS</pre></body><!-- Mirrored from c-faq.com/struct/align.esr3.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:28 GMT --></html>⌨️ 快捷键说明
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